Vehicle steering apparatus

ABSTRACT

An electric motor provided in a vehicle steering apparatus includes a stator coaxially surrounding a steering shaft, an inner rotor disposed on the inner periphery of the stator, and an outer rotor disposed on the outer periphery of the stator. The facing area between the stator and the rotor can be increased by providing the inner rotor and the outer rotor in the electric motor. Consequently, the electric motor can be miniaturized while a certain output thereof is maintained. The vehicle steering apparatus can be miniaturized by miniaturizing the electric motor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vehicle steering apparatus for use ina vehicle, such as an automobile.

2. Related Art

Some vehicle steering apparatus for use in a vehicle, such as anautomobile, uses an electric motor to assist a driver's steeringoperation. For example, the following JP-A-2005-145436 has proposed whatis called a direct drive vehicle steering apparatus in which an outputtorque of an electric motor is transmitted directly to a steering shaftwithout the intermediation of a reduction mechanism.

Vehicle steering apparatuses are disposed in a vehicle interior that isa limited space. Thus, preferably, the vehicle steering apparatuses arecompact.

On the other hand, in vehicle steering apparatuses of the direct drivetype, an output torque of an electric motor is not amplified by areduction mechanism. Accordingly, the vehicle steering apparatuses ofthe direct drive type require a high output electric motor. Therefore,the vehicle steering apparatuses of the direct drive type often use alarge electric motor. This contradicts the request for theminiaturization of the vehicle steering apparatuses.

SUMMARY OF THE INVENTION

The invention is accomplished against such a background. An object ofthe invention is to miniaturize a vehicle steering apparatus.

To achieve the foregoing object, according to the invention, there isprovided a vehicle steering apparatus (1, 1 a, 1 b, 1 c, 1 d, 1 e)having an electric motor (4, 104, 204, 304), provided coaxially with asteering shaft (5, 54, 55, 57), for giving a steering force to thesteering shaft. In the vehicle steering apparatus, the electric motorincludes an annular stator (25) fixed to a vehicle-body-side member(100), a cylindrical inner rotor (26) configured to face an innerperiphery of the stator and to be rotatable around an axis line (L1) ofthe steering shaft, and a cylindrical outer rotor (27) configured toface an outer periphery of the stator and to be rotatable around theaxis line of the steering shaft.

According to the invention, the inner rotor and the outer rotor aredisposed along the inner periphery and the outer periphery of thestator, respectively. Therefore, as compared with an electric motorhaving only one of the inner rotor and the outer rotor, the electricmotor according to the invention can increase the facing area betweenthe stator and the rotor. Consequently, an output of the electric motorcan be increased. Accordingly, the electric motor can be miniaturizedwhile a certain output is maintained. The miniaturization of theelectric motor results in the miniaturization of the vehicle steeringapparatus. Thus, the mountability of the vehicle steering apparatus to avehicle can be enhanced.

Further, the rotor is smaller than the stator in the volume forgenerating a magnetic force of the same magnitude. Thus, the electricmotor can be further miniaturized by providing a pair of rotors therein.

A steering shaft connected to a steering member, such as a steeringwheel, can be used as the steering shaft. In a case where arack-and-pinion mechanism is used as a steering mechanism for steering asteered wheel, a pinion shaft can be used as the steering shaft.Alternatively, a rack shaft can be used as the steering shaft.Alternatively, another shaft can be used as the steering shaft.

That is, the vehicle steering apparatus according to the invention canbe what is called a column-assist type one. Alternatively, the vehiclesteering apparatus according to the invention can be what is called apinion-assist type one. Alternatively, the vehicle steering apparatusaccording to the invention can be what is called a rack-assist type one.The vehicle steering apparatus according to the invention can beconfigured so that the steering member and the steering mechanism aremechanically connected to each other. Alternatively, the vehiclesteering apparatus according to the invention can be a vehicle steeringapparatus (what is called a steer-by-wire type one) configured so thatthe steering member and the steering mechanism are not mechanicallyconnected to each other.

The outer rotor may be used also as the housing of the electric motor.In this case, the electric motor can be further miniaturized. Inaddition, the electric motor can be reduced in weight.

The stator may be sandwiched between the inner cylinder (34) and theouter cylinder (35), which are fixed to the vehicle-body-side member. Inthis case, the stator is sandwiched between the inner cylinder and theouter cylinder, which are fixed to the vehicle-body-side member. Thus,the stator can securely be fixed to the vehicle-body-side member.Consequently, an output of the electric motor can be stabilized.

The stator may include a stator core (129) which includes an annularyoke (31), and a plurality of teeth (32) formed on an inner periphery ofthe yoke so as to be projected therefrom and as to be arranged in acircumferential direction (Y1) of the stator at intervals so that a coil(30) is wound around each of the plurality of teeth. An end portion ofthe yoke is located in an axial direction of the stator in the yoke andincludes a first extension part (47) extended in the axial direction(X2) of the stator. An end portion of each of the teeth is located inthe axial direction of the stator therein and includes a secondextension part (48) extended in the axial direction of the stator. Thefirst extension part and the second extension part are fixed to thevehicle-body-side member. In this case, the first and second extensionparts, which are a part of the stator core, are fixed to thevehicle-body-side member. Thus, the stator can securely be fixed to thevehicle-body-side member. Consequently, an output of the electric motorcan be stabilized.

In the foregoing description, reference numerals in parenthesesdesignate associated components of embodiments that will be describedlater. However, the reference numerals in the parentheses have not beendescribed for purposes of limitation of the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative cross-sectional view illustrating theschematic configuration of an electric power steering apparatusaccording to an embodiment of the invention.

FIG. 2 is an illustrative enlarged cross-sectional view of a part of theelectric power steering apparatus according to the embodiment of theinvention.

FIG. 3 is an illustrative cross-sectional view of a part of an electricmotor, which is taken along line III-III shown in FIG. 1.

FIG. 4 is an illustrative enlarged cross-sectional view of a part of anelectric power steering apparatus according to another embodiment of theinvention.

FIG. 5 is an illustrative enlarged cross-sectional view of a part of anelectric power steering apparatus according to still another embodimentof the invention.

FIG. 6 is an illustrative view illustrating the appearance of a statorcore provided in the electric power steering apparatus according to thestill another embodiment of the invention.

FIG. 7 is a schematic view illustrating the schematic configuration ofan electric power steering apparatus according to a yet anotherembodiment of the invention.

FIG. 8 is a schematic view illustrating the schematic configuration ofan electric power steering apparatus according to a further anotherembodiment of the invention.

FIG. 9 is a schematic view illustrating the schematic configuration of avehicle steering apparatus according to a still another embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the invention are described withreference to the accompanying drawings.

FIG. 1 is an illustrative cross-sectional view illustrating theschematic configuration of an electric power steering apparatusaccording to an embodiment of the invention.

Referring to FIG. 1, an electric power steering apparatus 1 serving as avehicle steering apparatus includes a steering member 2, such as asteering wheel, a steering mechanism 3 for steering steered wheels (notshown) in response to a steering operation of the steering member 2, andan electric motor 4 for steering assist. For example, a rack-and-pinionmechanism is used as the steering mechanism 3.

The steering member 2 and the steering mechanism 3 are mechanicallyconnected to each other via a steering shaft 5. The steering operation(rotation) of the steering member 2 is transmitted to the steeringmechanism 3 via the steering shaft 5. Then, the rotation transmitted tothe steering mechanism 3 is converted into an axial movement of a rackshaft (not shown). Consequently, the steered wheel is steered.

The steering shaft 5 linearly extends. The electric motor 4 is coaxiallyconnected to the steering shaft 5. In the present embodiment, thesteering shaft 5 functions as a steering shaft. That is, the electricpower steering apparatus 1 according to the present embodiment is avehicle steering apparatus of what is called a column assist type.Hereinafter, a part at the side of the steering member 2 along an axialdirection X1 of the steering shaft 5 is referred to as an upper side,while a part at the side of the steering mechanism 3 thereof is referredto a lower side.

The steering shaft 5 includes an input shaft 6 and an output shaft 7.The input shaft 6 and the output shaft 7 are coaxially connected to eachother via a torsion bar 8. The input shaft 6 and the output shaft 7 canperform a relative rotation.

The input shaft 6 includes an upper shaft 9 and a lower shaft 10. Thesteering member 2 is corotatably connected to the top portion of theupper shaft 9. The output shaft 7 is connected to the bottom portion ofthe lower shaft 10 via the torsion bar 8.

The upper shaft 9 is cylindrical. The top portion of the lower shaft 10is, for example, serration-fit into the inner periphery of the bottomportion of the upper shaft 9. The upper shaft 9 and the lower shaft 10are connected to each other so as to be corotatable around the centralaxis line L1 of the steering shaft 5 and as to be able to perform arelative rotation in the axial direction X1 of the steering shaft 5.

The steering shaft 5 passes through the inner periphery of a cylindricalsteering column 11. The steering shaft 5 is supported rotatably aroundthe steering column 11. The steering column 11 includes a cylindricalupper tube 12 and a cylindrical lower tube 13, which are fit to eachother.

Each of the upper tube 12 and the lower tube 13 extends in the axialdirection X1. The upper tube 12 and the lower tube 13 are disposedcoaxially with each other. The upper tube 12 and the lower tube 13 areconnected to each other so as to perform a relative movement in an axialdirection (the same direction as the axial direction X1 illustrated inFIG. 1) of the steering column 11. More specifically, the top portion ofthe lower tube 13 is fit into the inner periphery of the upper tube 12.

The inner peripheral surface of the upper tube 12 and the outerperipheral surface of the lower tube 13 are, for example, frictionallyengaged with each other on a fitting portion on which the upper tube 12and the lower tube 13 are fit to each other. Accordingly, when arelative movement in the axial direction X1 between the upper tube 12and the lower tube 13 is caused, resistance is imposed to one or both ofthe upper tube 12 and the lower tube 13.

The upper tube 12 supports the upper shaft 9 via a bearing 60 a so thatthe upper shaft 9 can rotate. The upper shaft 9 and the upper tube 12are movably connected to each other as to be able to move together inthe axial direction X1 by the bearing 60 a. The lower tube 13 includes afirst cylindrical tube 14 and a second cylindrical tube 15, which arefit to each other. The first tube 14 and the second tube 15 are fixed toeach other. The first tube 14 is disposed above the second tube 15 inthe axial direction X1. The second tube 15 includes a cylindricalportion 16 extending along the axial direction X1, and includes also anannular flange portion 17 protruding radially outwardly from the outerperipheral surface of the cylindrical portion 16.

The upper tube 12 is connected to the first tube 14. The second tube 15supports the top portion of the output shaft 7 via a bearing 60 b sothat the output shaft 7 can rotate. A torque sensor 18 for detecting asteering torque is disposed on the inner periphery of the second tube15. More particularly, a torque sensor 18 is disposed at a positioncorresponding to a connecting portion, at which the input shaft 6 andthe output shaft 7 are connected, on the inner periphery of the secondtube 15.

The torque sensor 18 detects a steering torque, based on arelative-rotation displacement amount between the input shaft 6 and theoutput shaft 7. A result of detection of a torque, which is detected bythe torque sensor 18, is input to an electronic control unit (ECU) 19.

The steering column 11 is fixed to a vehicle-body-side member 100 with apredetermined fixing strength by a first bracket 20 fixed to the outerperiphery of the upper tube 12. More specifically, the first bracket 20is fixed to the vehicle-body-side member 100 via, for example, asynthetic resin pin 21.

When an impact of a magnitude having a value equal to or greater than apredetermined value is applied to the first bracket 20, for example,upon a vehicle collision, the pin 21 is fractured. Thus, the fixationbetween the upper tube 12 and the vehicle-body-side member 100 isreleased. Accordingly, the steering member 2, the upper shaft 9, and theupper tube 12 move together downwardly in the axial direction X1 of thevehicle-body-side member 100. At that time, the upper tube 12 movesdownwardly in the axial direction X1 to the lower tube 13. Resistanceacting in a direction opposite to the direction, in which the upper tube12 moves, is imposed to the upper tube 12 from the lower tube 13.Consequently, an impact imposed to the steering member 2 from a driverdue to what is called a secondary collision is absorbed.

The steering shaft 5 is rotatably supported by the vehicle-body-sidemember 100 via the bearing 60 c under the electric motor 4. Moreparticularly, the bottom portion of the output shaft 7 is supported byan annular bearing holding member 22 via a bearing 60 c. The bearingholding member 22 is fixed to the vehicle-body-side member 100 via asecond bracket 23. The second bracket 23 is fixed to thevehicle-body-side member 100 with a fixing bolt 24.

Next, the electric motor 4 is described below in detail.

FIG. 2 is an illustrative enlarged cross-sectional view of a part of theelectric power steering apparatus according to the embodiment of theinvention. FIG. 3 is an illustrative cross-sectional view of a part ofthe electric motor 4, which is taken along line III-III shown in FIG. 1.

Referring to FIG. 2, the electric motor 4 according to the presentembodiment is a brushless motor. The electric motor 4 includes anannular stator 25 coaxially surrounding the steering shaft 5, acylindrical inner rotor 26 facing the inner periphery of the stator 25across a gap, and a cylindrical outer rotor 27 facing the outerperiphery of the stator 25 across a gap. The inner rotor 26 and theouter rotor 27 coaxially surround the steering shaft 5. The stator 25,the inner rotor 26, and the outer rotor 27 are housed in a cylindricalmotor housing 28.

Referring to FIGS. 2 and 3, the stator 25 includes an annular statorcore 29, and a plurality of coils 30 wound around the stator core 29.More specifically, the stator core 29 includes a cylindrical yoke 31constituting an outer peripheral portion of the stator core 29, and aplurality of teeth 32 protruding from the inner peripheral surface ofthe yoke 31 in a radially inward direction of the yoke 31.

The plurality of teeth 32 are arranged at equal intervals in acircumferential direction Y1 of the stator core 29. Each of the teeth 32is cross-sectionally T-shaped. More particularly, each of the teeth 32includes a radial extension portion 32 a extended in a radially inwarddirection of the yoke 31 from the inner peripheral surface of the yoke31, and a circumferential extension portion 32 b that is connected to aninner end portion of the radial extension portion 32 a and that extendsalong a circumferential direction of the yoke 31.

Each of the teeth 32 has a length substantially equal to that in theaxial direction X1 of the yoke 31. The plurality of coils 30 are woundaround the plurality of teeth 32, respectively. Each of the coils 30 iselectrically connected to the ECU 19. Electrical power is supplied toeach of the coils 30 from an electric power supply (not shown) via adrive circuit (not shown) of the ECU 19.

The stator core 29 includes a plurality of arcuate split cores 33. Thatis, the yoke 31 includes a plurality of split elements 31 a obtained bydividing the yoke 31 in the circumferential direction Y1. Each of theteeth 32 is formed in an associated one of the split elements 31 a. Eachof the split cores 33 includes an associated one of the split elements31 a and an associated one of the teeth 32. The plurality of split cores33 are circularly combined with one another. Incidentally, the statorcore 29 can be of the integrated type that is not divided in thecircumferential direction Y1.

Referring to FIG. 2, the stator core 29 is held by an inner cylinder 34disposed on the inner periphery thereof, and an outer cylinder 35disposed on the outer periphery thereof. Each of the inner cylinder 34and the outer cylinder 35 is a cylindrical member which extends alongthe axial direction X1 and which is longer in the length in the axialdirection X1 than the stator 25. Each of the inner cylinder 34 and theouter cylinder 35 is made of, for example, a metal.

The inner cylinder 34 is fixed to the inner periphery of the stator core29 by being, for example, pressed or shrinkage-fit thereinto. The statorcore 29 is held between the inner cylinder 34 and the outer cylinder 35by being sandwiched therebetween.

An annular support member 36 is fixed to the bottom portions of theinner cylinder 34 and the outer cylinder 35 by, for example, welding oradhesion. The support member 36 coaxially surrounds the output shaft 7.The inside diameter and the outside diameter of the support member 36are substantially equal to those of the cylindrical portion 16 of thesecond tube 15. The support member 36 supports the output shaft 7 via abearing 60 d so that the output shaft 7 can rotate.

The top portions of the inner cylinder 34 and the outer cylinder 35 arefixed to the bottom portion of the cylindrical portion 16 of the secondtube 15 by, for example, welding or adhesion. The stator 25 is fixed tothe vehicle-body-side member 100 via the inner cylinder 34 and the outercylinder 35. That is, the second tube 15, to which the inner cylinder 34and the outer cylinder 35 are fixed, is fixed to the first tube 14. Thefirst tube 14 is fixed to the upper tube 12. The upper tube 12 is fixedto the vehicle-body-side member 100 via the first bracket 20.

The inner rotor 26 includes a plurality of permanent magnets 37 arrangedcircularly. Each of the permanent magnets 37 is substantially equal inthe length in the axial direction X1 to the stator 25. An outer surfaceof each of the permanent magnets 37 is fixed to the outer peripheralsurface of the output shaft 7 at a position facing the inner peripheryof the stator 25. The plurality of permanent magnets 37 are arranged inthe circumferential direction of the steering shaft 5. The polarities ofthe magnet poles of each pair of adjacent ones of the permanent magnets37 differ from each other. Magnets poles including alternately arrangedN-poles and S-poles are formed on the outer periphery of the inner rotor26. Each permanent magnet 37 and the output shaft 7 can rotate togetheraround the central axis line L1 of the steering shaft 5. When anelectromagnetic force generated by the stator 25 is applied to eachpermanent magnet 37, a rotating torque is directly transmitted to theoutput shaft 7.

The outer rotor 27 includes a cup-like main part 39 having a cylindricalperipheral wall portion 38, and includes also a plurality of permanentmagnets 40 held on the inner periphery of the peripheral wall portion38. The stator 25 and the inner rotor 26 are disposed on the innerperiphery of the main part 39.

The peripheral wall portion 38 of the main part 39 extends along theaxial direction X1 and is longer in the length in the axial direction X1than the stator 25. The peripheral wall portion 38 coaxially surroundsthe output shaft 7. An annular end wall 41 extending from the bottom ofthe peripheral wall portion 38 in a radially inward direction of theperipheral wall portion 38 is provided on the bottom of the peripheralwall portion 38. The inner peripheral surface of the end wall 41 isfixed to the outer peripheral surface of the output shaft 7. The endwall 41 and the output shaft 7 are rotatably connected to each other.That is, the main part 39 and the output shaft 7 are connected to beable to corotate around the central axis line L1 of the steering shaft5.

The peripheral wall portion 38 is such that the bottom portion thereofis rotatably supported by the support member 36 via the bearing 60 e,and that the top portion thereof is rotatably supported by thecylindrical portion 16 of the second tube 15 via a bearing 60 f. Theperipheral wall portion 38 is supported at two places that are locatedat a distance in the axial direction X1. Thus, the peripheral wallportion 38 can stably rotate without run-out. Consequently, a stablerotating torque is transmitted from the electric motor 4 to the outputshaft 7.

The rotational position of the peripheral wall portion 38 is detected bya resolver 42 serving as a rotational position detecting device. Theresolver 42 includes an annular resolver rotor 43, which is fixed to theinner peripheral surface of the peripheral wall portion 38 above thebearing 60 f supporting the top portion of the peripheral wall portion38, and includes also a resolver stator 44 fixed to the outer peripheralsurface of the cylindrical portion 16 of the second tube 15 above thebearing 60 f.

The resolver 42 can detect the rotational position of the outer rotor 27by detecting the rotational position of the peripheral wall portion 38.Because the outer rotor 27, the output shaft 7, and the inner rotor 26rotate together, the resolver 42 can detect the rotational position ofthe inner rotor 26 by detecting the rotational position of theperipheral wall portion 38. A result of detection of the rotationalpositions of the outer rotor 27 and the inner rotor 26, which aredetected by the resolver 42, is input to the ECU 19. Incidentally, therotational position detecting device according to the present embodimentis not limited to the resolver 42. Another detecting device, such as arotary encoder, can be used as the rotational position detecting device.

Each permanent magnet 40 is substantially equal in the length in theaxial direction X1 to the stator 25. Each permanent magnet 40 is fixedto the inner peripheral surface of the peripheral wall portion 38 at aposition at which an inner surface thereof faces the outer peripheralsurface of the stator 25. The plurality of permanent magnets 40 arearranged in the circumferential direction of the peripheral wall portion38. The polarities of the magnet poles of each pair of adjacent ones ofthe permanent magnets 40 differ from each other. Magnets poles includingalternately arranged N-poles and S-poles are formed on the innerperiphery of the outer rotor 27. Each permanent magnet 40 and the mainpart 39 can corotate. When an electromagnetic force generated by thestator 25 is applied to each permanent magnet 40, a rotating torque isdirectly transmitted to the output shaft 7 via the peripheral wallportion 38 and the end wall 41.

A motor housing 28 includes a cylindrical portion 45 coaxiallysurrounding the output shaft 7, and includes also an annular end wall 46extending from the bottom of the cylindrical portion 45 in a radiallyinward portion of the cylindrical portion 45. The stator 25, the innerrotor 26, and the outer rotor 27 are disposed on the inner periphery ofthe cylindrical portion 45.

The cylindrical portion 45 extends along the axial direction X1 and islonger in the length in the axial direction X1 than the outer rotor 27.The cylindrical portion 45 surrounds the outer rotor 27 radially acrossa gap. The inner peripheral surface of the top part of the cylindricalportion 45 is fixed to the outer peripheral surface of the flangeportion 17 of the second tube 15. A bearing 60 g intervenes between theinner peripheral surface of the end wall 46 and the outer peripheralsurface of the output shaft 7. The end wall 46 supports the output shaft7 via the bearing 60 g so that the output shaft 7 can rotate.

The electric motor 4 is controlled by the ECU 19. That is, the ECU 19controls the electric motor 4, based on a steering torque input to thesteering member 2, on the rotational positions of the inner rotor 26 andthe outer rotor 27, and on a vehicle speed input from a vehicle speedsensor (not shown). Consequently, a rotating torque serving as asteering assist force is transmitted from the electric motor 4 directlyto the output shaft 7. Accordingly, the steering assist force istransmitted to the steering mechanism 3 so as to assist a driver'ssteering operation.

As described above, in the present embodiment, the inner rotor 26 andthe outer rotor 27 are disposed in the inner periphery and the outerperiphery of the stator 25, respectively. Therefore, as compared with anelectric motor having only one of the inner rotor and the outer rotor,the facing area between the inner rotor and the outer rotor can beincreased. Consequently, an output of the electric motor 4 can beincreased. Accordingly, the electric motor 4 can be miniaturized while acertain output is maintained. The miniaturization of the electric motor4 results in the miniaturization of the electric power steeringapparatus 1. Thus, the mountability of the electric power steeringapparatus 1 to a vehicle can be improved. The rotor is smaller than thestator in the volume needed to generate a magnetic force of the samemagnitude. Consequently, the electric motor 4 can be furtherminiaturized by providing a pair of rotors therein.

Additionally, in the present embodiment, the stator 25 is fixed to theinner cylinder 34 and the outer cylinder 35. Thus, the stator 25 cansecurely be fixed to the vehicle-body-side member 100. Consequently, thevibration of the stator 25 can be prevented. The stable rotating torquecan be transmitted from the electric motor 4 to the output shaft 7.Because the inner cylinder 34 and the outer cylinder 35 have simpleshapes (cylindrical shapes), the manufacturing cost of these cylinderscan be suppressed.

That is, in a case where the stator 25 is embedded in a synthetic resinmember (what is called a resin mold) without using the inner cylinder 34and the outer cylinder 35 and where this synthetic resin member is fixedto the second tube 15, the stator 25 cannot securely be fixed to thesecond tube 15 due to the low stiffness of the synthetic resin member.Further, in a case where the stator 25 is fixed to a metal cage statorholding member, and where this stator holding member is fixed to thesecond tube 15, the stator 25 can securely be fixed to thevehicle-body-side member 100. However, the manufacturing cost of thestator holding member is high due to the complex shape thereof. Thus,these problems can be solved by fixing the stator 25 to thevehicle-body-side member 100 using the inner cylinder 34 and the outercylinder 35.

FIG. 4 is an illustrative enlarged cross-sectional view of a part of anelectric power steering apparatus 1 a according to another embodiment ofthe invention. In FIG. 4, a component equivalent to each part shown inFIG. 2 is designated with the same reference numeral as that used todenote the equivalent part shown in FIG. 2. Thus, the description ofsuch components is omitted.

Referring to FIG. 4, the embodiment illustrated in FIG. 4 differs fromthat illustrated in FIG. 2 mainly in that the main part 39 of the outerrotor 27 is used also as a motor housing. Additionally, a bearing with aseal (for example, a rolling ball bearing with double seals) is used asthe bearing 60 h intervening between the top portion of the peripheralwall portion 38 of the main part 39 and the cylindrical portion 16 ofthe second tube 115. The second tube 115 includes the cylindricalportion 16.

In the present embodiment, the main part 39 is used also as a motorhousing. Thus, the electric motor 4 can be reduced in size and weight.Further, foreign substances, such as grit and dust, can be prevented byusing a bearing with a seal as the bearing 60 h from entering theelectric motor 4. Consequently, the durability of the electric motor 4can be enhanced.

FIG. 5 is an illustrative enlarged cross-sectional view of a part of anelectric power steering apparatus 1 b according to still anotherembodiment of the invention. FIG. 6 is an illustrative view illustratingthe appearance of a stator core 129 provided in the electric powersteering apparatus 1 b according to the still another embodiment of theinvention. In FIGS. 5 and 6, a component equivalent to each part shownin FIGS. 2 and 3 is designated with the same reference numeral as thatused to denote the equivalent part shown in FIGS. 2 and 3. Thus, thedescription of such components is omitted.

Referring to FIG. 5, the embodiment illustrated in FIG. 5 differs fromthat illustrated in FIG. 2 mainly in that the inner cylinder 34 and theouter cylinder 35 shown in FIG. 2 are not provided in the embodimentillustrated in FIG. 5, and that the stator core 129 is fixed directly tothe second tube 15. Such differences are described more specifically inthe following description.

Referring to FIG. 6, each of parts of both end portions in an axialdirection X2 of the stator core 129 is extended in the axial directionX2 of the stator core 129. That is, a couple of extension parts (i.e., afirst extension part 47 and a second extension part 48) are provided atboth end portions of the stator core 129, respectively.

More particularly, both end portions in the axial direction X2 of theyoke 31 are extended in the axial direction X2 and constitute the firstextension part 47. Both end portions in the axial direction X2 of thecircumferential extension part 32 b, which is a part of the teeth 32,are extended in the axial direction X2 and constitute the secondextension part 48. The first extension part 47 and the second extensionpart 48 are annular and face each other radially across a gap. The firstextension part 47 coaxially surrounds the second extension part 48across a gap in a radial direction of the stator core 129.

Referring to FIG. 5, a couple of annular step portions are formed at thebottom part of the cylindrical portion 16 of the second tube 15. Acouple of annular concave portions 51 and 50 are formed between thebottom part of the cylindrical portion 16 and the bearing 60 b disposedon the inner periphery of the bottom part thereof and between the bottompart of the cylindrical portion 16 and the bearing 60 f disposed on theouter periphery of the bottom part thereof, respectively. The firstextension part 47 and the second extension part 48, which are providedat the upper side in the axial direction X2, are, for example, press-fitinto the couple of the annular concave portions 50 and 51, respectively.Consequently, the stator core 129 is fixed to the second tube 15. Thestator core 129 is fixed to the vehicle-body-side member 100 via thesecond tube 15.

A couple of annular step portions are formed at the top portion of thesupport member 36. A couple of annular concave portions 53 and 52 areformed between the top portion of the support member 36 and the bearing60 d disposed on the inner periphery of this top portion and between thetop portion of the support member 36 and the bearing 60 e disposed onthe outer periphery of this top portion, respectively. The firstextension part 47 and the second extension part 48, which are providedat a lower side in the axial direction X2, are, for example, press-fitinto a couple of the annular concave portions 52 and 53, respectively.Consequently, the support member 36 is fixed to the stator core 129.

In the present embodiment, each of the first extension part 47 and thesecond extension part 48, which is a part of the stator core 129, isfixed to the second tube 15. Thus, the stator core 129 can securely befixed to the second tube 15. That is, the stator 25 can securely befixed to the second tube 15. Consequently, a stable rotating torque canbe transmitted from the electric motor 4 to the output shaft 7.According to the present embodiment, the number of components can bereduced, as compared with the embodiment illustrated in FIG. 2.

Incidentally, FIGS. 5 and 6 illustrate a case where the first extensionpart 47 is integrally formed of the yoke 31 and a single member. Thefirst extension part 47 according to the invention is not limitedthereto. The first extension part 47 can be formed of the yoke 31 andanother member and can be fixed to the yoke 31. Similarly, FIGS. 5 and 6illustrate a case where the second extension part 48 is integrallyformed of the teeth 32 and a single member. The second extension part 48according to the invention is not limited thereto. The second extensionpart 48 can be formed of the teeth 32 and another member and can befixed to the teeth 32.

The invention is not limited to the aforementioned embodiments. Variousmodifications can be made within the scope of the invention, which isdescribed in claims. For instance, in the embodiment illustrated in FIG.2, an example of fixing the inner cylinder 34 and the outer cylinder 35to the second tube 15 by welding or adhesion has been described.However, similarly to the embodiment illustrated in FIG. 5, the innercylinder 34 and the outer cylinder 35 can be fixed to the second tube 15by press-fitting. Similarly, the support member 36 can be fixed to theinner cylinder 34 and the outer cylinder 35 by press-fitting.

In the embodiment illustrated in FIG. 5, an example of fixing the statorcore 129 to the second tube 15 by press-fitting has been described.However, the stator core 129 can be fixed to the second tube 15 bywelding or adhesion. Similarly, the support member 36 can be fixed tothe stator core 129 by welding or adhesion.

In the aforementioned embodiments, an example of applying the inventionto what is called a column-assist type vehicle steering apparatus hasbeen described. However, the vehicle steering apparatus according to theinvention is not limited to that of the column-assist type. The vehiclesteering apparatus according to the invention can be what is called apinion-assist type vehicle steering apparatus. Alternatively, thevehicle steering apparatus according to the invention can be what iscalled a rack-assist type vehicle steering apparatus. Alternatively, thevehicle steering apparatus according to the invention can be what iscalled a steer-by-wire type vehicle steering apparatus in which asteering member and a steering mechanism are not mechanically connectedto each other.

More specifically, an electric motor 104 including a stator, an innerrotor and an outer rotor can coaxially be connected to a pinion shaft 54of a pinion-assist type electric power steering apparatus 1 c, asillustrated in FIG. 7. In this case, the pinion shaft 54 functions as asteering shaft.

An electric motor 204 including a stator, an inner rotor and an outerrotor can coaxially be provided on a rack shaft 55 of a rack-assist typeelectric power steering apparatus 1 d, as illustrated in FIG. 8. Inaddition, a motion conversion mechanism 56, for example a ball screw,for converting a rotation of an electric motor 204 into an axialmovement of a rack shaft 55 can intervene between the electric motor 204and the rack shaft 55. In this case, the rack shaft 55 functions as asteering shaft.

An electric motor 304 including a stator, an inner rotor and an outerrotor can coaxially be provided on a steering shaft 57 of asteer-by-wire type vehicle steering apparatus 1 e, as illustrated inFIG. 9. In addition, a motion conversion mechanism 56, for example aball screw, can intervene between the electric motor 304 and thesteering shaft 57.

1. A vehicle steering apparatus having an electric motor, providedcoaxially with a steering shaft, forgiving a steering force to saidsteering shaft, wherein said electric motor includes an annular statorfixed to a vehicle-body-side member, a cylindrical inner rotorconfigured to face an inner periphery of said stator and to be rotatablearound an axis line of said steering shaft, and a cylindrical outerrotor configured to face an outer periphery of said stator and to berotatable around the axis line of said steering shaft.
 2. The vehiclesteering apparatus according to claim 1, wherein said outer rotor isserved as a housing of said electric motor.
 3. The vehicle steeringapparatus according to claim 1, wherein said stator is sandwichedbetween an inner cylinder and an outer cylinder, which are fixed to saidvehicle-body-side member.
 4. The vehicle steering apparatus according toclaim 1, wherein said stator includes a stator core which includes anannular yoke, and a plurality of teeth formed on an inner periphery ofsaid yoke so as to be projected therefrom and as to be arranged in acircumferential direction of said stator at intervals so that a coil iswound around each of said plurality of teeth; an end portion of saidyoke is located in an axial direction of said stator in said yoke andincludes a first extension part extended in the axial direction of saidstator; an end portion of each of said teeth is located in the axialdirection of said stator therein and includes a second extension partextended in the axial direction of said stator; and said first extensionpart and said second extension part are fixed to said vehicle-body-sidemember.